1. Field of the Invention
An exemplary aspect of the present invention relates to a synthetic aperture radar for achieving a compact polarimetric SAR for transmitting a circular polarization or a linear polarization and receiving vertical and horizontal polarizations, using a general-purpose phased array antenna for vertical and horizontal polarizations.
2. Description of the Related Art
Data of a synthetic aperture radar (SAR) with respect to a transmission polarization and a reception polarization can be formed by combining a horizontal polarization (an H polarization) and a vertical polarization (a V polarization), to be four kinds of polarization states such as a horizontal-horizontal polarization (HH), a horizontal-vertical polarization (HV), a vertical-vertical polarization (VV), and a vertical-horizontal polarization (VH).
With the above four kinds of polarization data, a reflection polarizational characteristics of an observed object can be reproduced completely. To obtain four kinds of the polarizations simultaneously, a full polarimetric SAR is generally required. In the full polarimetric SAR, the horizontal polarization and the vertical polarization are transmitted alternately at every pulse, and both of the polarizations are received simultaneously, to obtain four kinds of polarization data.
Particularly in a satellite SAR, a swath width and a data amount with the full polarimetric SAR become problems. Therefore, the compact polarimetric SAR has been proposed recently. The compact polarimetric SAR performs transmission by using a circular polarization or a 45 degrees linear polarization, and performs reception of two polarizations of the horizontal polarization and the vertical polarization. The compact polarimetric SAR uses reflection characteristics of radar and performs a statistical processing so as to reproduce four kinds of the polarization states. A method using this technique is described in “Compact Polarimetry Based on Symmetry Properties of Geophysical Media: The π/4 Mode” by Jean-Claude Souyris, Roger Fjrtoft, Sandra Mingot, and Jong-Sen Lee, in IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, 2005 (Non-Patent Document 1), for example. Further, “Hybrid-Polarity SAR Architecture” by R. Keith Raney, in IGARSS' 2006 (Non-Patent Document 2), discloses a structure for achieving a compact poralimetric SAR by using a commonly-used antenna.
However, in the compact polarimetric SARs disclosed in Non-Patent Documents 1 and 2, different kinds of polarizations are required to be used, that is, the circular polarization or the 45 degrees linear polarization is required for transmission, and the horizontal polarization and the vertical polarization are required for reception, and therefore a problem is that a structure of the SAR becomes complicated.
Further, the structure becomes more complicated when it is intended to be applied to a conventional full polarimetric SAR or SARs using a single polarization such as the horizontal-horizontal (HH) polarization, the vertical-vertical (VV) polarization and the like. That ends up a problem in which mass grows, loss in antenna increases, and reliability degreases.
Furthermore, a technique in Japanese Patent Application Laid-open No. 2006-322848 (Patent Document 1) is for a full polarimetric SAR which transmits a horizontal polarization (H) and a vertical polarization (V) alternately. However, the way of achieving an SAR is completely different from that of the compact polarimetric SAR in the present invention, which transmits the horizontal polarization (H) and the vertical polarization (V) simultaneously.
Moreover, Japanese Patent Application Laid-open No. 2002-064321 (Patent Document 2) discloses a structure in which a third array antenna having a given polarization characteristic and a fourth array antenna having a polarizational characteristic orthogonal to the polarizational characteristic of the third array antenna are combined, and Japanese Patent Application Laid-open No. 03-089606 (Patent Document 3) discloses a structure in which all polarizations are set in the vertical or the horizontal polarization by setting a relative phase to be in a same phase. However, in order to use different kinds of polarizations, namely using the circular polarization or the 45 degrees linear polarization in transmission and using the horizontal polarization and the vertical polarization in reception, it is required that an H polarization radiating element and a V polarization radiating element, which share a phase center, are to be fed simultaneously for transmission, and such a problem causes the structure of a compact polarimetric SAR to be complicated.